Lysine is the first limiting amino acid (AA) in typical swine diets, and plays important roles in nutrient metabolism and growth performance of pigs. This research was conducted to study the effects of dietary lysine on blood plasma concentrations of protein, carbohydrate and lipid metabolites of pigs. Eighteen crossbred (Yorkshire × Landrace) finishing barrows and gilts (9/sex; BW 92.3 ± 6.9 kg) were individually penned in an environment-controlled barn. Pigs were assigned to 3 dietary treatments according to a randomized complete block design with sex as block and pig as experimental unit (6 pigs/treatment). Three corn and soybean-meal based diets were formulated to contain total lysine at 0.43, 0.71, and 0.98% (as-fed basis) for diets 1 (D1; lysine-deficient), 2 (D2; lysine-adequate), and 3 (D3; lysine-excess), respectively. After 4 weeks on trial, jugular vein blood was collected and plasma separated. The plasma concentrations of glucose, total protein, albumin, triglyceride, urea nitrogen (PUN), and total cholesterol were determined using an ACE Clinical Chemistry System (Alfa Wassermann, Inc.). Data were analyzed using the GLM Proc with pdiff (adjust = T) option of SAS. No differences were found between barrows and gilts in any of the measured metabolites. The plasma concentration of PUN was greater for pigs fed D1 than D3 (P < 0.05), and greater for pigs fed D3 than D2 (P < 0.05). The plasma albumin concentration was lower for pigs fed D1 than D2 or D3 (P < 0.05), and no difference between D2 and D3. The plasma total cholesterol concentration was greater in pigs fed D1 than D2 or D3 (P < 0.05), and no difference between D2 and D3. The plasma concentration of total protein, glucose or triglyceride did not differ among the 3 treatments. These results confirm that a diet deficient in one essential AA can lead to catabolism of other AAs and impair body protein syntheses, and that the excess amount of one AA over the requirement will be catabolized. However, the metabolic mechanism for the increased plasma total cholesterol concentration of finishing pigs by a lysine-deficient diet warrants further investigation. This research was supported by USDA-NIFA Hatch/Multistate Project 233803.